1. Field of the Invention
The present invention relates to suspension systems for off-road vehicles, such as those used in agricultural and construction businesses, and more particularly to adjustable hydraulically controlled suspension systems.
2. Description of the Related Art
Off-road vehicles, such as construction and agricultural equipment, carry a wide range of loads over different types of surfaces. Thus, the suspension systems between the wheels and the frame of these vehicles are important for optimum operator comfort, productivity and safety. The suspension system must be adjustable depending upon the weight of the load applied to the equipment. A very heavy load forces the vehicle body downward with respect to the axles, which can adversely affect the maneuverability of the vehicle. On the other hand, if the suspension is configured for relatively heavy loads, the vehicle may have an undesirable ride under lighter loads.
As a result, many vehicles have automatic load leveling systems which employ one or more hydraulic cylinders between the axle and the frame of the vehicle to ensure that the frame is maintained at the proper height above the axle. When a heavy load is applied to the frame, the drop of the frame is sensed and additional hydraulic fluid is applied to the cylinder to raise the frame to a desired distance from the axle. When that heavy load is removed from the vehicle, the frame rises significantly above the axle. When this occurs hydraulic fluid is drained from the cylinder to lower the frame with respect to the axle. This type of automatic hydraulic load leveling system ensures that the frame and axle are maintained at the desired separation regardless of the size of the load applied to the vehicle.
The hydraulic cylinder also functions as a shock absorber by regulating the flow of fluid from a chamber on one side of the piston to the chamber on the other side as the vehicle bounces when traveling over the ground. Although this shock absorbing action is beneficial to creating a smoother ride and greater operator comfort, there are times when it is necessary to lock-out the front axle to produce a “solid axle”. For example, when the vehicle is carrying a heavy load there may not be a need for shock absorber action of the suspension as the tire perform that task. Also the shock absorber action is not required at relatively low speeds.
The lock-out function in prior suspension systems employed a plurality of electrically operated hydraulic valves to block the flow of fluid between the cylinder chambers in additional to flow between each chamber and sources of fluid, such as a pump and an accumulator. This multiple valve arrangement added significantly to the expense of the suspension.
Therefore, it is desirable to provide a less expensive lock-out mechanism which does not adversely affect the performance characteristics of the suspension system.